Method and apparatus for space division multiple access for wireless local area network system

ABSTRACT

A space division multiple access (SDMA) method is provided. The method may be performed by a station (STA), in a wireless local area network (WLAN) system. The method includes overhearing a third transmission response message transmitted by an access point (AP), and determining whether to access a channel on the basis of criteria information included in the third transmission response message.

CROSS-REFERENCE TO RELATED APPLICATIONS

This application is a continuation of U.S. application Ser. No.14/174,649, filed on Feb. 6, 2014, now U.S. Pat. No. 9,161,260, which isa continuation of U.S. application Ser. No. 13/321,531, filed on Nov.18, 2011, now U.S. Pat. No. 8,681,793, which is the National Stagefiling under 35 U.S.C. 371 of International Application No.PCT/KR2010/003196, filed on May 20, 2010, which claims the benefit ofU.S. Provisional Application No. 61/180,443, filed on May 22, 2009, thecontents of which are incorporated by reference herein in theirentirety.

BACKGROUND OF THE INVENTION

Field of the Invention

The present invention relates to wireless communications, and moreparticularly, to a method of space division multiple access (SDMA) in awireless local area network (WLAN) system and an apparatus forsupporting the method.

Discussion of the Background Art

With the advancement of information communication technologies, variouswireless communication technologies have recently been developed. Amongthe wireless communication technologies, a wireless local area network(WLAN) is a technology whereby Internet access is possible in a wirelessfashion in homes or businesses or in a region providing a specificservice by using a portable terminal such as a personal digitalassistant (PDA), a laptop computer, a portable multimedia player (PMP),etc.

Ever since the institute of electrical and electronics engineers (IEEE)802, i.e., a standardization organization for WLAN technologies, wasestablished in February 1980, many standardization works have beenconducted. In the initial WLAN technology, a frequency of 2.4 GHz wasused according to the IEEE 802.11 to support a data rate of 1 to 2 Mbpsby using frequency hopping, spread spectrum, infrared communication,etc. Recently, the WLAN technology can support a data rate of up to 54Mbps by using orthogonal frequency division multiplex (OFDM). Inaddition, the IEEE 802.11 is developing or commercializing standards ofvarious technologies such as quality of service (QoS) improvement,access point protocol compatibility, security enhancement, radioresource measurement, wireless access in vehicular environments, fastroaming, mesh networks, inter-working with external networks, wirelessnetwork management, etc.

The IEEE 802.11n is a technical standard relatively recently introducedto overcome a limited data rate which has been considered as a drawbackin the WLAN. The IEEE 802.11n is devised to increase network speed andreliability and to extend an operational distance of a wireless network.More specifically, the IEEE 802.11n supports a high throughput (HT),i.e., a data processing rate of up to 540 Mbps or higher, and is basedon a multiple input and multiple output (MIMO) technique which usesmultiple antennas in both a transmitter and a receiver to minimize atransmission error and to optimize a data rate. In addition, thisstandard may use a coding scheme which transmits several duplicatecopies to increase data reliability and also may use the OFDM to supporta higher data rate.

With the widespread use of the WLAN and the diversification ofapplications using the WLAN, there is a recent demand for a new WLANsystem to support a higher throughput than a data processing ratesupported by the IEEE 802.11n. However, an IEEE 802.11n medium accesscontrol (MAC)/physical layer (PHY) protocol is not effective to providea throughput of above 1 Gbps. This is because the IEEE 802.11n MAC/PHYprotocol is designed for an operation of a station (STA), that is, anSTA having one network interface card (NIC), and thus when a framethroughput is increased while conforming to the conventional IEEE802.11n MAC/PHY protocol, a resultant additional overhead is alsoincreased. Consequently, there is a limitation in increasing athroughput of a wireless communication network while conforming to theconventional IEEE 802.11n MAC/PHY protocol, that is, a single STAarchitecture.

Therefore, to achieve a data processing rate of above 1 Gbps in thewireless communication system, a new system different from theconventional IEEE 802.11n MAC/PHY protocol (i.e., the single STAarchitecture) is required. A very high throughput (VHT) WLAN system is anext version of the IEEE 802.11n WLAN system, and is one of IEEE 802.11WLAN systems which have recently been proposed to support a dataprocessing rate of above 1 Gbps in a MAC service access point (SAP).

The VHT WLAN system allows simultaneous channel access of a plurality ofVHT STAs for the effective use of a radio channel. For this, amulti-user multiple input multiple output (MU-MIMO)-based transmissionusing multiple antennas is supported. The VHT AP can perform spatialdivision multiple access (SDMA) transmission for transmitting spatiallymultiplexed data to the plurality of VHT STAs.

SUMMARY OF THE INVENTION

The present invention provides a space division multiple access (SDMA)method capable of optimizing resource utilization of a wireless mediumin a wireless local area network (WLAN) system.

The present invention also provides a wireless communication apparatussupporting an SDMA method capable of optimizing resource utilization ofa wireless medium in a WLAN system.

In an aspect, a space division multiple access (SDMA) method, performedby a station (STA), in a wireless local area network (WLAN) system isprovided. The method includes overhearing a third transmission responsemessage transmitted by an access point (AP), and determining whether toaccess a channel on the basis of criteria information included in thethird transmission response message.

The third transmission response message may be a response for a thirdtransmission request message transmitted by a third STA to the AP, andThe criteria information may be channel state information acquired bythe AP by estimating the channel on the basis of the third transmissionrequest message.

The determining of whether to access to the channel on the basis of thecriteria information may include determining a channel correlation levelof the STA and the third STA on the basis of the channel stateinformation, and deferring channel access if the channel correlationlevel is higher than a specific level, and continuing channel access ifthe channel correlation level is lower than the specific level.

The third transmission request message may be a request to send (RTS)frame or a sounding physical layer convergence procedure (PLCP) protocoldata unit (PPDU).

The third transmission response message may be a clear to send (CTS)frame.

The third transmission response message may be a response for the thirdtransmission request message transmitted by the third STA to the AP.

The criteria information may be an access category of traffic to betransmitted by the third STA and is included in the third transmissionrequest message received by the AP.

The determining of whether to access the channel on the basis of thecriteria information may include determining whether the access categoryof the traffic to be transmitted by the STA is equal to an accesscategory indicated by the criteria information, and deferring channelaccess if the access category of the traffic to be transmitted by theSTA is different from the access category indicated by the criteriainformation, and continuing channel access if the access category of thetraffic to be transmitted by the STA is equal to the access categoryindicated by the criteria information.

The third transmission response message may be a response for the thirdtransmission request message transmitted by the third STA to the AP.

The criteria information may be permissible received power level rangeinformation determined on the basis of a received power level of thethird transmission request message for the AP.

The determining of whether to access the channel on the basis of thecriteria information may include determining whether the received powerlevel of the third transmission response message for the STA fallswithin a received power level range indicated by the criteriainformation, and deferring channel access if the received power level ofthe third transmission response message for the STA is out of thereceived power level range indicated by the criteria information, andcontinuing channel access if the received power level of the thirdtransmission response message for the STA falls within the receivedpower level range indicated by the criteria information.

According to the present invention, space division multiple access(SDMA) transmission is achieved for stations (STAs) grouped on the basisof a channel correlation or a target received power level, and thusutilization efficiency of a radio resource can be improved andtransmission efficiency can also be improved.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a schematic view showing an exemplary structure of a wirelesslocal area network (WLAN) system to which an embodiment of the presentinvention can be applied.

FIG. 2 is a flowchart showing a channel access method of a station (STA)according to an embodiment of the present invention.

FIG. 3 shows an example of a space division multiple access (SDMA)channel access method according to an embodiment of the presentinvention.

FIG. 4 shows an example of an SDMA method applicable to downlinktransmission according to the present invention.

FIG. 5 is a block diagram showing an exemplary format of a targetindication message according to an embodiment of the present invention.

FIG. 6 is a block diagram showing a wireless apparatus for implementingan embodiment of the present invention.

DETAILED DESCRIPTION OF THE INVENTION

Hereinafter, embodiments of the present invention will be described withreference to the accompanying drawings. The embodiments of the presentinvention described below can be effectively applied to a very highthroughput (VHT) wireless local area network (WLAN) system supportingmulti user multiple input multiple output (MU-MIMO). Although the VHTWLAN system will be described hereinafter for example, the technicalfeatures of the present invention are not limited thereto. For example,the embodiments of the present invention can be also equally applied tovarious wireless communication systems using a method of determiningwhether one or more stations (STAs) will access a channel in an adaptivemanner according to a channel state.

FIG. 1 is a schematic view showing an exemplary structure of a WLANsystem to which an embodiment of the present invention can be applied.

Referring to FIG. 1, the WLAN system includes one or more basis servicesets (BSSs). The BSS is a set of stations (STAs) which are successfullysynchronized to communicate with one another, and is not a conceptindicating a specific region. The BSS can be classified into aninfrastructure BSS and an independent BSS (IBSS). The infrastructure BSSis shown in FIG. 1. Infrastructure BSSs (i.e., BSS1 and BSS2) includeone or more STAs (i.e., STA1, STA3, and STA4), access points (APs) whichare STAs providing a distribution service, and a distribution system(DS) connecting a plurality of APs (i.e., AP1 and AP2). On the otherhand, the IBSS does not include APs, and thus all STAs are mobile STAs.In addition, the IBSS constitutes a self-contained network sinceconnection to the DS is not allowed.

The STA is an arbitrary functional medium including a medium accesscontrol (MAC) and wireless-medium physical layer interface conforming tothe institute of electrical and electronics engineers (IEEE) 802.11standard, and includes both an AP and a non-AP STA in a broad sense. AVHT STA is defined as an STA that supports super high-rate dataprocessing of above 1 GHz in the multi-channel environment to bedescribed below. In the VHT WLAN system to which the embodiment of thepresent invention is applicable, STAs included in the BSS may be all VHTSTAs, or a VHT STA and a legacy STA (i.e., IEEE 802.11n-based HT STA)may coexist.

Among the STAs, non-AP STAs (i.e., STA1, STA3, and STA4) are portableterminals operated by users. A non-AP STA may be simply referred to asan STA. The non-AP STA may also be referred to as a wirelesstransmit/receive unit (WTRU), a user equipment (UE), a mobile station(MS), a mobile terminal, a mobile subscriber unit, etc. A non-AP VHT-STA(or simply a VHT STA) is defined as a non-AP STA that supports the superhigh-speed data processing of above 1 GHz and space division multipleaccess (SDMA) to be described below.

The AP (i.e., AP1 and AP2) is a functional entity for providingconnection to the DS through a wireless medium for an associated STA.Although communication between non-AP STAs in an infrastructure BSSincluding the AP is performed via the AP in principle, the non-AP STAscan perform direct communication when a direct link is set up. Inaddition to the terminology of an access point, the AP may also bereferred to as a centralized controller, a base station (BS), a node-B,a base transceiver system (BTS), a site controller, etc. A VHT AP isdefined as an AP that supports the super high-speed data processing ofabove 1 GHz and SDMA to be described below.

A plurality of infrastructure BSSs can be interconnected by the use ofthe DS. An extended service set (ESS) is a plurality of BSSs connectedby the use of the DS. STAs included in the ESS can communicate with oneanother. In the same ESS, a non-AP STA can move from one BSS to anotherBSS while performing seamless communication.

The DS is a mechanism whereby one AP communicates with another AP. Byusing the DS, an AP may transmit a frame for STAs associated with a BSSmanaged by the AP, or transmit a frame when any one of the STAs moves toanother BSS, or transmit a frame to an external network such as a wirednetwork. The DS is not necessarily a network, and has no limitation inits format as long as a specific distribution service specified in theIEEE 802.11 can be provided. For example, the DS may be a wirelessnetwork such as a mesh network, or may be a physical structure forinterconnecting APs.

Hereinafter, transmission from an AP to an STA is referred to asdownlink transmission, and transmission from the STA to the AP isreferred to as uplink transmission. The AP and the STA may transmitspatially multiplexed data in uplink transmission or downlinktransmission through multiple antennas. Transmission of the spatiallymultiplexed data to a plurality of STAs is referred to as SDMAtransmission. To perform SDMA transmission by allowing the plurality ofSTAs to simultaneously access a channel, the STAs may simultaneouslyperform transmission over a plurality of spatial streams by usingrespective multiple antennas.

When using an SDMA method in which the plurality of STAs perform uplinktransmission or downlink reception with an AP by simultaneouslyaccessing a channel, the STAs participating in transmission bysimultaneously accessing the channel need to be coordinated according toa specific criterion in order to optimize utilization of a radioresource. As an example, STAs having a low channel correlation betweenthe AP and the respective STAs participating in transmission may begrouped for the effective use when participating in transmission. Thisis because, when STAs having a high correlation participate intransmission simultaneously, an interference level may increase and thusa performance gain may decrease.

The present invention proposes an SDMA method for increasing aperformance gain by coordinating STAs participating in transmissionsimultaneously according to a specific criterion.

FIG. 2 is a flowchart showing a channel access method of an STAaccording to an embodiment of the present invention.

The STA acquires criteria information before channel access (step S210).The criteria information is a criterion for determining whether the STAwill continue or defer channel access. The STA may acquire the criteriainformation from other STAs in addition to an AP. A method of acquiringthe criteria information may be a method of overhearing a frametransmitted by the AP to other STAs as well as directly receiving thecriteria information which is broadcast or unicast by the AP. Examplesof the criterion for determining whether to continue the access includea channel correlation, an access category (AC), a received power level,etc., each of which will be described below in detail together with amethod of acquiring the criterion.

The STA determines whether to continue channel access on the basis ofthe criteria information acquired in step S210 (step S220). In thiscase, a detailed method used in the determination may vary according toa type of the criteria information, and will be described below togetherwith specific embodiments.

If the STA continues channel access, the STA transmits a transmissionrequest message to the AP (step S240). The transmission request messagemay be used when the STA reports to the AP that a frame to betransmitted is present or when the AP estimates a channel between the APand the STA. The transmission request message may be a newly definedmanagement action frame, or may be a sounding physical layer convergenceprocedure (PLCP) protocol unit (PPDU) or a request to send (RTS) framebased on the IEEE 802.11 standard.

Thereafter, the STA receives a transmission response message from the APin response to the transmission request message (step S250). Thetransmission response message may include the criteria information or aresult obtained when the AP performs channel estimation by receiving thetransmission request message. The transmission response message may be anewly defined management action frame or a clear to send (CTS) framebased on the IEEE 802.11 standard. A third STA accessing a channelafterwards may overhear the transmission response message and determinewhether to continue channel access on the basis of the criteriainformation included in the transmission response message.

Thereafter, the STA performs uplink transmission or downlink receptiontogether with other STAs simultaneously accessing the channel (stepS260).

If the determination result of step S220 based on the criteriainformation shows that it is not proper to continue channel access, theSTA defers channel access (step S270).

Hereinafter, a specific example of the criteria information and an SDMAchannel access method related thereto will be described in detail.

According to an embodiment of the present invention, a criterion fordetermining whether an STA will continue channel access may be a channelcorrelation. In this case, the criteria information is channel stateinformation between an AP and each STA.

STAs, which desire to access a channel in uplink SDMA, attempt channelaccess according to a channel access method (e.g., a backoff mechanismof enhanced distributed channel access (EDCA)) based on the IEEE 802.11standard. An STA, of which a backoff timer expires first, transmits atransmission request message to an AP. The AP receives the transmissionrequest message and performs channel estimation. The AP transmits atransmission response message to the STA. In this case, the transmissionresponse message includes channel state information known to the AP inaddition to a result of the channel estimation. STAs, which overhear thetransmission response message, can determine whether to continue channelaccess on the basis of the channel state information included in thetransmission response message. For example, if the channel stateinformation indicates that a channel correlation is high between an STAwhich has requested the AP to perform uplink transmission and an STAwhich attempts channel access, the STA which attempts channel access maydefer channel access.

From the perspective of the STA which attempts channel access, the STAwhich currently attempts channel access acquires the channel stateinformation by overhearing the transmission response message transmittedin response to the transmission request message of the STA which haspreviously accessed the channel, and determines whether to access thechannel on the basis of the channel state information. The channel stateinformation included in the transmission response message may beutilized as information for decreasing a channel correlation among STAssimultaneously participating in uplink transmission.

FIG. 3 shows an example of an SDMA channel access method according to anembodiment of the present invention.

In the example of FIG. 3, an STA1 310, an STA2 320, an STA3 330, and anSTA4 340 perform uplink transmission to an AP 300. It is shown in theexample of FIG. 3 that four STAs simultaneously attempt channel access,and use an RTS frame as a transmission request message and a CTS frameas a transmission response message. However, this is for one exemplaryembodiment of the present invention only, and thus the number of STAsattempting channel access is not limited to that exemplified in FIG. 3.In addition, the transmission request message is not constrained to theRTS frame, and the transmission response message is not constrained tothe CTS frame. As described above, the RTS frame of FIG. 3 may be anewly defined frame or a sounding PPDU and the CTS frame may be a newlydefined frame, and the same will be true hereinafter.

The STA1 310, the STA2 320, the STA3 330, and the STA4 340 attemptchannel access in order to participate in uplink transmission. A backofftimer of the STA 310 expires first, and the STA 310 transmits an RTSframe 311 to the AP 300. The AP 300 receives the RTS frame 311, andbased on this, performs channel estimation with respect to the STA1 310.The AP 300 transmits a CTS frame 302 as a response. In this case, theCTS frame 302 includes not only a channel estimation result but alsochannel state information of STAs known to the AP 300. The STA2 320, theSTA3 330, and the STA4 340 overhear the CTS frame 302, and determinewhether to continue channel access on the basis of the channel stateinformation included in the CTS frame 302.

Thereafter, backoff timers of the STA2 320 and the STA3 330 expire, andthe STA2 320 and the STA3 330 transmit an RTS frame 323 and an RTS frame334, and the AP 300 transmits a CTS frame 303 and a CTS frame 304 inresponse thereto. The CTS frame 303 may further include a channelestimation result with respect to the STA2 320. The CTS frame 304 mayfurther include a channel estimation result with respect to the STA3330. In the example of FIG. 3, the STA2 320 has a channel correlationhaving a specific level or lower with respect to the STA1 310 whichfirst accesses a channel on the basis of the channel state informationacquired from the CTS frame 302. The STA2 320 participates in uplinkSDMA transmission by accessing the channel. Similarly, the STA3 330 hasa channel correlation having a specific level or lower with respect tothe STA1 310 and STA2 320 which first access a channel on the basis ofthe channel state information acquired from the CTS frame 302 or the CTSframe 303. The STA3 330 participates in uplink SDMA transmission byaccessing the channel.

Unlike the STA2 320 and the STA3 330, the STA4 340 has a high channelcorrelation with respect to at least one of the STA1 310, the STA2 320,and the STA3 330 on the basis of the channel state information acquiredby overhearing the CTS frame 304. The STA4 340 defers channel access. Asan example of deferring channel access, the STA4 340 may configure anetwork allocation vector (NAV).

The SDMA method according to the embodiment of the present invention canalso be applied to downlink transmission.

In downlink SDMA, an AP broadcasts a target indication message. Thetarget indication message includes information on a target STA of aframe stored in a buffer of the AP and to be transmitted in downlink. Asan example, the target indication message may include a MAC address listof the target STA. Upon receiving the target indication message, STAsdetermine whether they are target STAs. Similarly to the aforementioneduplink transmission, the target STA performs a channel estimationprocess by exchanging a transmission request message and a transmissionresponse message. In this case, the transmission request message mayinclude a result of channel estimation performed by the STA by receivingthe target indication message.

The transmission request message/transmission response message can beexchanged between the target STAs and the AP according to an orderdesignated by the AP. For this, the AP may transmit backoff timerconfiguration information of the STA. The STA may sequentially exchangethe transmission request message/transmission response message with theAP by configuring a backoff timer according to the backoff timerconfiguration information of the STA. As an example, if an STA_A, anSTA_B, and an STA_C are target STAs, the AP transmits values of 1, 2,and 3 respectively to the STA_A, the STA_B, and the STA_C as the backofftimer configuration information. Upon receiving the backoff timerconfiguration information, the STA_A, the STA_B, and the STA_C mayrespectively set their backoff timers to 1, 2, and 3 so that thetransmission request message/transmission response message are exchangedin sequence.

As another example, a backoff mechanism may be used to exchange thetransmission request message/transmission response message. In a casewhere the STA_A, the STA_B, and the STA_C decrease their backoff timersin order to perform channel access, if the backoff timer of the STA_Aexpires first and the exchange of the transmission requestmessage/transmission response message starts, the STA_B and the STA_C donot reconfigure their backoff timers but maintain the backoff timersconfigured when the STA_A starts the exchange of the transmissionrequest message/transmission response message, and decrease the backofftimers after the STA_A exchanges the transmission requestmessage/transmission response message.

Similarly to the aforementioned uplink SDMA, the transmission responsemessage may include channel state information. One STA knows a channelcorrelation with respect to another STA which accesses a channel priorto the STA itself on the basis of the channel state information, andaccordingly may defer or continue channel access.

FIG. 4 shows an example of an SDMA method applicable to downlinktransmission according to the present invention.

An AP 400 broadcasts an SDMA poll frame 401 as a target indicationmessage. The SDMA poll frame 401 includes a list of target STAs forreceiving SDMA data 407 to be transmitted in a downlink. In the exampleof FIG. 4, the SDMA poll frame 401 includes information indicating anSTA1 410, an STA2 420, an STA3 430, and an STA4 440, and may be MACaddresses of the STA1 410, the STA2 420, the STA3 430, and the STA4 440.Upon receiving the SDMA poll frame 401, the STA1 410, the STA2 420, theSTA3 430, and the STA4 440 exchange RTS/CTS frames with the AP 400. TheRTS frame is an example of a transmission request message, and the CTSframe is an example of a transmission response message. The RTS/CTSframes may be exchanged between the AP 400 and the STAs (i.e., the STA1410, the STA2 420, the STA3 430, and the STA4 440) on the basis ofbackoff timer configuration information transmitted by the AP 400 asdescribed above, or may be exchanged according to a backoff timer expiryorder determined by random backoff.

The STA4 440 overhears a CTS frame 403 to acquire channel stateinformation included in the CTS frame 403, and thus can determinewhether to continue channel access. In the example of FIG. 4, the STA4440 has a high channel correlation with respect to at least one STAamong the STA1 410, the STA2 420, and the STA3 430. In this case, theSTA4 440 defers channel access.

Regarding the STA1 410, the STA2 420, and the STA3 430 having a lowchannel correlation, the AP 400 transmits the spatially multiplexed data407 to the STA1 410, the STA2 420, and the STA3 430. As such, SDMAtransmission can be simultaneously performed on a group of STAs having alow channel correlation, and thus usage of radio resources can beoptimized.

FIG. 5 shows an exemplary format of a target indication messageaccording to an embodiment of the present invention.

A target indication message 500 includes an action category field 510,an action value field 520, and a target STA address field 530.

The action category field 510 and the action value field 520 brieflyindicate an operational description and a category of a specific frame.That is, the specific frame is related to indication of a target STA,and whether to include additional information containing a list oftarget STAs can be indicated.

The target STA address field 530 includes identification information oftarget STAs. The identification information of the target STAs may beMAC addresses or AIDs of the target STAs.

The target indication message 500 may further include a priority rankfield 540. The priority rank field may include information fordesignating an order when respective target STAs exchange a transmissionrequest message/transmission response message with an AP. As an example,backoff timer configuration information of each target STA may beincluded. Each target STA may configure its backoff timer on the basisof the backoff timer configuration information, and may exchange thetransmission request message/transmission response message with the APaccording to a backoff timer expiry order. That is, an STA having apriority rank of 0 may set its backoff timer to 0 and an STA having apriority rank of 4 may set its backoff timer to 4, and then an order ofexchanging the transmission request message/transmission responsemessage may be determined between the STAs.

According to another embodiment of the present invention, criteriainformation may be an access category (AC). The AC may be informationindicating a category of a frame to be transmitted or informationindicating a transmission priority. The AC may be included in an EDCAparameter set, and may be used as a label for indicating a transmissionpriority or a data type of a frame including the EDCA parameter set.

TABLE 1 Value AC Description 00 AC_BE Best effort 01 AC_BK Background 10AC_VI Video 11 AC_VO Voice

Table 1 shows an example of the AC. In addition to AC_BE, AC_BK, AC_VI,and AC_VO of Table 1, another AC type may be added or some of the ACtypes may be omitted according to a configuration.

When the AC is used as the criteria information, an AP may transmit atransmission response frame by inserting AC information into thetransmission response frame so that only STAs having traffic of aspecific AC are allowed for access. Alternatively, an STA may insert ACinformation of uplink traffic desired to be transmitted by the STA to atransmission request frame to be transmitted to the AP. The AP acquiresthe AC information from the received transmission request frame, andretransmits the transmission request message by inserting the ACinformation into the transmission response message.

Referring back to the example of FIG. 3, the STA1 310 transmits the RTSframe 311 by inserting AC information of traffic to be transmitted bythe STA1 310 into the RTS frame 311. The AP 300 piggybacks the ACinformation included in the RTS frame 311 to the CTS frame 302. The STA2320, the STA3 330, and the STA4 340 determine whether to continuechannel access on the basis of the AC information of the CTS frame 302.

If the AP 300 sets an AC to AC_VI in the CTS frame 302, an STA havingtraffic corresponding to AC_BE, AC_BK, and AC_VO other than AC_VI deferschannel access. Accordingly, only STAs having traffic corresponding toAC_VI can participate in uplink transmission by transmitting RTS frames.

According to another embodiment of the present invention, the criteriainformation may be a received power level. A channel state or a distancebetween the AP and each STA may be different, and thus received powerlevels of uplink transmission frames simultaneously received by the APfrom STAs participating in uplink transmission may be different from oneanother. To increase a performance gain of uplink SDMA transmission,received power levels of respective STAs' transmission frames receivedby the AP need to be set to similar values. Respective STAs' transmitpower may be regulated so that the received power levels of therespective STAs' transmission frames received by the AP are set tosimilar values, but in this case, if a specific STA decreases itstransmit power for this reason, there is a problem in that a modulationand coding scheme (MCS) also has to be selected to a low level.Therefore, there is a need for a method in which an STA does notdecrease it's transmit power and an AP simultaneously receives traffichaving a similar received power level.

This problem can be solved according to the embodiment of the presentinvention in which the received power level is used as the criteriainformation. By using the received power level of the AP as a criterionfor determining whether the STA will continue channel access, the AP canparticipate in uplink transmission by accessing a channel for each groupby grouping STAs having the similar received power level (i.e., onlySTAs having the similar received power level simultaneously access thechannel).

Referring back to the example FIG. 3, the AP 300 transmits a CTS frameincluding an accessible target received power level. The target receivedpower level may be set to a range of a received power level at which theAP will allow access. The range of the received power level at which theAP will allow access may be set on the basis of a received power levelmeasured by receiving the RTS frame 311 from the STAT 310. Uponoverhearing the CTS frame, the STA2 320, the STA3 330, and the STA4 340determine whether a received power level for the CTS frame is within areceived power level range (MIN, MAX) included in the CTS frame, and ifthe determination result is affirmative, continues channel access, andotherwise, differs channel access.

Although the channel correlation, the access category, and the receivedpower level are taken as an example of the criterion for determiningwhether the STA will continue channel access in the aforementioneddescription, various criteria may be included in the criteriainformation according to a configuration. The various criteria fordetermining whether the STA will continue channel access may be combinedand used together.

FIG. 6 is a block diagram showing a wireless apparatus for implementingan embodiment of the present invention. A wireless apparatus 600 may bean AP or an STA.

The wireless apparatus 600 includes a processor 610, a memory 620, and atransceiver 630. The transceiver 630 may have a plurality of networkinterface cards (NICs), and may transmit/receive a frame through aplurality of antennas by using an SDMA method. The processor 610 iscoupled to the transceiver 630 and performs SDMA based on the methodproposed in the present invention. Further, the processor 610 generatesa frame for the SDMA and processes a received frame. The processor 610and the transceiver 630 implement a physical layer and a MAC layer ofIEEE 802.11. The processor 610 and/or the transceiver 630 may include anapplication-specific integrated circuit (ASIC), a separate chipset, alogic circuit, and/or a data processing unit. The memory 620 may includea read-only memory (ROM), a random access memory (RAM), a flash memory,a memory card, a storage medium, and/or other equivalent storagedevices. When the embodiment of the present invention is implemented insoftware, the aforementioned methods can be implemented with a module(i.e., process, function, etc.) for performing the aforementionedfunctions. The module may be stored in the memory 620 and may beperformed by the processor 610. The memory 620 may be located inside oroutside the processor 610, and may be coupled to the processor 610 byusing various well-known means.

The aforementioned embodiments include various exemplary aspects.Although all possible combinations for representing the various aspectscannot be described, it will be understood by those skilled in the artthat other combinations are also possible. Therefore, all replacements,modifications and changes should fall within the spirit and scope of theclaims of the present invention.

What is claimed:
 1. A method for performing a channel access in awireless local area network, the method comprising: receiving, by anaccess point (AP), a plurality of status messages from a plurality ofstations, each status message includes about an access category (AC)indicating a quality of service for an uplink traffic that acorresponding station wants to send; transmitting, by the AP, an accessmessage to the plurality of stations for requesting the plurality ofstations to participate uplink transmissions; and receiving, by the AP,a plurality of data frames from the plurality of stations, wherein theaccess message includes information about a plurality of ACs for theplurality of stations, each AC indicating a recommended quality ofservice for an uplink traffic that is to be sent by a correspondingstation.
 2. The method of claim 1, wherein the access message furtherincludes information about a plurality of target received signal powersfor the plurality of stations.
 3. The method of claim 1, wherein each ACin the access message is represented as two bits.
 4. A device forperforming a channel access in a wireless local area network, the devicecomprising: a transceiver configured to transmit and receive radiosignals; and a processor operatively coupled with the transceiver andconfigured to: instruct the transceiver to receive a plurality of statusmessages from a plurality of stations, each status message includesabout an access category (AC) indicating a quality of service for anuplink traffic that a corresponding station wants to send; instruct thetransceiver to transmit an access message to the plurality of stationsfor requesting the plurality of stations to participate uplinktransmissions; and instruct the transceiver to receive a plurality ofdata frames from the plurality of stations, wherein the access messageincludes information about a plurality of ACs for the plurality ofstations, each AC indicating a recommended quality of service for anuplink traffic that is to be sent by a corresponding station.
 5. Thedevice of claim 4, wherein the access message further includesinformation about a plurality of target received signal powers for theplurality of stations.
 6. The device of claim 4, wherein each AC in theaccess message is represented as two bits.